Dimers and trimers of formamidine and its mono-halogenated analogues HN@CHNHX, (X = H, Cl, Br, or I): A comparative study of resonance-assisted hydrogen and halogen bonds Ruben D. Parra ⇑ Department of Chemistry, DePaul University 1110 W. Belden Ave., Chicago, IL 60614, United States article info Article history: Received 17 May 2012 Received in revised form 29 June 2012 Accepted 18 July 2012 Available online 7 August 2012 Keywords: Halogen bonding Hydrogen bonding Formamidine Ab initio Density functional theory abstract The MP2 ab initio method, and the M052X and the B3LYP density functional theory methods were used to investigate the geometries and energetics of dimers and trimers of formamidine and its mono-haloge- nated analogues. The primary purpose of this study is to examine the strength of the resonance-assisted NAXN interactions for X = H (hydrogen bond), relative to that for X = Cl, Br, or I (halogen bond). It is found that, for the dimers and trimers studied here, the hydrogen bond interaction is stronger than the halogen bond interactions when the halogen is either Cl or Br. For dimers, the NAHN interaction is actually of comparable strength to that of the NAIN interaction. For example, at the MP2 level the NAHN interaction energy is 7.47 kcal/mol, and the corresponding NAIN interaction energy is found to be 7.13 kcal/mol. Trimerization produces a small decrease in the hydrogen bond strength, with a hydrogen bond energy of 6.90 kcal/mol at the MP2 level. The opposite is true for the halogen bonds which actually get stronger upon trimer formation, not all to the same extent though. At the MP2 level, for instance, the magnitudes of the interaction energies increase by factors of 1.28 and 1.98 for NAClN and NABrN respectively. A much more substantial increase factor of 3.36 is found in the NAIN inter- action energy upon trimer formation. The strengthening of the NAIN interaction is so dramatic that the iodine is found midway between the two nitrogen atoms, and the interaction in the trimer is better described as a symmetric NIN interaction. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction The pivotal role that non-covalent interactions play in chemis- try, biochemistry, and materials science is well recognized by the scientific community at large [1–4]. Without a doubt, the most investigated of these interactions is the highly directional hydro- gen bond interaction. In the almost 100 years since its inception in the literature [5], the term hydrogen bond has been the subject of a great many books, see for example [6–10] for some current contributions, and a myriad of research publications that can be seen, for example, in a number of recent review articles [11–19]. Despite the long history and the many publications using the term hydrogen bond, it should be noted that the lack of a universally ac- cepted understanding of what constitutes a hydrogen bond has been the source of some controversies. See for example the reviews on blue-shifting in hydrogen bonds [15,16], or the issue of weak and non-conventional hydrogen bonds [8,17,18]. To provide a common conceptual framework to discuss the hydrogen bond, an IUPAC Technical Report was released very recently where the following definition is recommended [20,21]: The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment XAH in which X is more electronegative than H, and an atom or a group of atoms in the same or a different mole- cule, in which there is evidence of bond formation. The report was re- leased by a Task Group charged with categorizing hydrogen bonding and other intermolecular interactions. The Task Group considered what is currently known, experimentally and theoreti- cally, about the hydrogen bond as a basis for the recommended definition. The Task Group also acknowledges that the criteria and characteristics used to define a hydrogen bond are likely to evolve with the advent of new theoretical and experimental tech- niques. In fact, in a recent review, Grabowski discusses the issue of covalency in hydrogen bonding using geometry, energetic, natural bond orbital, and atoms in molecules or topological analysis crite- ria [19]. Sánchez-Sanz et al. have recently proposed the use of fragment based method to estimate electron density shifts to investigate non-bonding intramolecular interactions including hydrogen bonds, NBr, halogen–halogen interactions among oth- ers [22]. Mo has just published a paper questioning whether the theory of atoms in molecules topological parameters can provide an effective measure of hydrogen bond strength [23]. 2210-271X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2012.07.032 ⇑ Tel.: +1 773 325 4343; fax: +1 773 325 7421. E-mail address: rparra1@depaul.edu Computational and Theoretical Chemistry 998 (2012) 183–192 Contents lists available at SciVerse ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc